Note: If you see this error message on major sites like Google, Facebook, Youtube and others in user accounts protected by Microsoft Family Safety filter after installing Windows Update KB2965142 (published on 6/9/2014), please refer to Microsoft's documentation on the subject: http://support.microsoft.com/kb/2965142/en-us#2

Certificates and identification

When you visit a website whose web address starts with https, your communication with the site is encrypted to help ensure your privacy. Before starting the encrypted communication, the website will present Firefox with a "certificate" to identify itself.

The certificate helps Firefox determine whether the site you're visiting is actually the site that it claims to be. If there is a problem with the certificate, you will see the This Connection Is Untrusted alert page.

Seeing the alert does not necessarily mean that the website you're visiting is trying to trick you into believing it is a different website - it means that Firefox isn't able to verify the identity of the website, and that you should proceed carefully.

There are several problems that can cause Firefox to reject a certificate. Some of them are described in detail in the Technical information section below.

Get out of there!

The safest thing to do is to click Get me out of here!, or to go to a different website. Unless you know and understand the technical reason why the website presented incorrect identification, and are willing to risk communicating over a connection that could be vulnerable to an eavesdropper, you should not proceed to the website.

If possible, you should contact the owners of the website and inform them of the error.

Technical information

Click on Technical Details for more information on why the website's identity information is invalid. Some common errors are described below.

Certificate will not be valid until (date)

(site name) uses an invalid security certificate. The certificate will not be valid until (date).
(Error code: sec_error_expired_issuer_certificate)

This error can occur if your computer clock has the wrong date, in which case the date given in the error message will be in the past. To fix the problem, set your system clock to today's date and time (double-click the clock icon on the Windows Taskbar).

Certificate is only valid for (site name)

This error is telling you that the identification sent to you by the site is actually for another site. While anything you send would be safe from eavesdroppers, the recipient may not be who you think it is.

A common situation is when the certificate is actually for a different part of the same site. For example, you may have visited https://example.com, but the certificate is for https://www.example.com. In this case, if you access https://www.example.com directly, you should not receive the warning.

The certificate is not trusted because the issuer certificate is unknown

(site name) uses an invalid security certificate. The certificate is not trusted because the issuer certificate is unknown.
(Error code: sec_error_unknown_issuer)

The file cert8.db in your profile folder may have become corrupted. Delete this file while Firefox is closed.

Open your profile folder:

At the top of the Firefox window, click on the Firefox button, go over to the Help menuOn the menu bar, click on the Help menuAt the top of the Firefox window, click on the Help menu and select Troubleshooting Information. The Troubleshooting Information tab will open.Click the menu button
, click help
and select Troubleshooting Information. The Troubleshooting Information tab will open.

Under the Application Basics section, click on Show FolderShow in FinderOpen Directory. A window with your profile filesfolder will open.

At the top of the Firefox window, click on the Firefox button and then select ExitAt the top of the Firefox window, click on the File menu and then select ExitOn the menu bar, click on the Firefox menu and select Quit FirefoxAt the top of the Firefox window, click on the File menu and select Quit

Click the menu button
and then click ExitQuit

Click on the file named cert8.db.

Press command+Delete.

Restart Firefox.

cert8.db will be recreated when you restart Firefox. This is normal.

The certificate is not trusted because no issuer chain was provided

(site name) uses an invalid security certificate. The certificate is not trusted because no issuer chain was provided.
(Error code: sec_error_unknown_issuer)

You may have enabled SSL scanning in your security software such as ESET or BitDefender. Try to disable this option.

The certificate is not trusted because it is self-signed

(site name) uses an invalid security certificate. The certificate is not trusted because it is self-signed.
(Error code: sec_error_untrusted_issuer)

Self-signed certificates make your data safe from eavesdroppers, but say nothing about who the recipient of the data is. This is common for intranet websites that aren't available publicly.

Bypassing the warning

You should only bypass the warning if you're confident in both the identity of the website and the integrity of your connection - even if you trust the site, someone could be tampering with your connection. Legitimate public sites will not ask you to add connection rule exceptions - an invalid certificate can be an indication of a web page that will defraud you or steal your identity.

If I Understand the Risks is missing then this page may be opened in an (i)frame and in that case try the right-click context menu and choose This Frame and then Open Frame in New Tab, and continue as above.